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RuntimeConfigBundle

This bundle provides a way to inject parameters into services at runtime by
exposing a RuntimeParameterBag service, which functions exactly like Symfony2's
own ParameterBags.

As-is, Symfony2's service container is compiled and cached to disk, which makes
it difficult to inject dynamic parameters. By exposing a ParameterBag service,
we can inject values returned from its get() method into other services.

One reason you might want support for dynamic parameters would be to implement
feature flags/flippers, as are used by GitHub and Flickr. More info on
the history behind this bundle may be found on the symfony-devs mailing list.

provider: A service implementing ParameterProviderInterface. If you are
using Doctrine ORM as your datasource, this could be an EntityRepository.

cascade: If true, calls to get() will cascade to the service container
when the parameter is undefined in the runtime configuration. This will
not change the behavior of has() or all(), which always only consider
parameters from the runtime configuration provider.

logging.enabled: Whether to enable logging access to undefined parameters,
regardless of whether service container cascading is enabled. If you are
using Monolog, logs will be sent to the "opensky.runtime_config" channel.

logging.level: Log level to use (should be a LoggerInterface method).

Note: when using cascade, it's a good idea to define default values for your
runtime configuration parameters in your service container. This will help
avoid an undesirable ParameterNotFoundException if you happen to fetch a
parameter that is not yet defined in your runtime configuration.

Injecting Parameters

Consider the scenario where "my.service" depends on a dynamic parameter
"my.service.enabled".

Runtime parameters may be conveniently injected by abusing the anonymous service
syntax in XML configurations:

Note: in both cases (anonymous and labeled services), Symfony2 requires that we
define the class on our service definition. The above examples use "stdClass" as
an arbitrary placeholder to satisfy CheckDefinitionValidityPass. In reality, our
service is simply a means to lazily load our parameter. The value returned byget() can be anything (e.g. object, scalar, array).

Cascade Mode

If you have enabled cascade mode, get() will attempt to fetch undefined
runtime parameters from the service container before throwing an exception.

In this example, get('my.services.enabled') would return false even if the
parameter was not defined in the runtime configuration. This is a safe way to
introduce new parameters, which might not yet be available from your provider
at the time of deployment.

Note: parameters sourced from the runtime configuration provider are not
resolved for placeholder syntax (i.e. "%reference%"), unlike those defined in
the service container.

Recipe: Interpreting Parameter Values as YAML

If you're using Doctrine ORM (or any database) to hold your parameters, you will
likely implement a CRUD interface to define and edit parameters via an admin
controller in your application.

Additionally, this allows us to add custom behavior to our ParameterProvider.
For instance, we can use Symfony2's YAML component to interpret parameter values
stored in the database as strings.

We must map an ID field, as the base Parameter class only defines essential
name and value fields.

The base class defines assertions for name and value fields (in groups, which
can be easily disabled); however, the mapped superclass does not define a
unique constraint on the name, so that is necessary.

A callback assertion is used to check that the value property is valid YAML.

The above Entity class is complemented by the following EntityRepository, which
serves as the ParameterProvider for the RuntimeParameterBag:

The base ParameterRepository already fetches name/value pairs from the database
via a DQL query. Using array_map(), we can easily interpret those values
through the same YAML component method.

Note: although we validate the Entity, it's possible that a value might have
been manually altered in the database and contain invalid YAML when parameters
are fetched for provision. If this is a concern, you may want to gracefully
handle thrown ParserExceptions within getParametersAsKeyValueHash().

Copyright (c) 2011 OpenSky Project Inc

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